Longa Strait under great pressure, which can some-
tive conditions that would result during an aver-
times present the greatest navigational obstacle of
age ice year with winds trending from the north to
the entire route. It was at this location that the 50-
northeast and prevailing throughout the winter
ship convoy became ice-bound in the autumn of
season. The winter season for the Bering Sea is
1983. The only midwinter area of weakness is a
considered to be six months long (December
lee-side polynya that forms off Wrangel Island that
through May). Winter in the Chukchi is nine
can occasionally be more than 50 nm wide. The
months long (November through July), and in the
prevailing onshore winds of winter normally shift
Beaufort Sea it lasts 10 months, beginning in Octo-
around to a more offshore flow in the spring, re-
ber and ending in July.
sulting in a narrow lane of open water along the
coast for the duration of the shipping season.
The Bering Sea
The summer melt pattern is primarily influ-
In the Bering Sea, open water can be expected
enced by the influx of warmer water from the
from about the Pribilof Islands southward. The
Bering Sea. Breakup initiates in the eastern end
southern limit of zone 1 marks the ice edge where
and progresses westward. At the height of sum-
pack ice in low concentrations is found. Accord-
mer (mid-September), the Chukchi is normally 80%
ing to the Alaska Marine Ice Atlas (LaBelle et al.
free of ice. Summer air temperatures average from
1983), zones 2 and 3 are areas of landfast ice with a
2 to 5C. Winter freezeup is usually delayed into
level-ice thickness range of 100 to 130 cm based on
September or October by this warmer inflow, and
freezing degree-day calculations (LaBelle et al.
open water north of the Bering Strait is found into
1983). Areas marked as zone 4 lie in the shadow of
late November.
Alaskan waters
The information presented in this sec-
tion was extracted primarily from the Arc-
tic Marine Transportation Program sum-
mary report by Voelker (1990). The
program, which took place from 1979 to
1986, collected field and operational data
during 12 deployments of a U.S. Polar-
class icebreaker to the ice-infested waters
surrounding Alaska. The program's pur-
pose was threefold:
To assess the feasibility of commer-
cial marine operations in ice,
To obtain data for improving ship
design for ice operations,
To define the environmental con-
ditions along potential shipping
routes.
Therefore, this environmental summary
contains information on the Chukchi,
Beaufort, and Bering Seas that is more ap-
plicable to ship trafficability than that pre-
sented for the Russian seas.
Based on an analysis of meteorological
and satellite imagery data and experience
gained from the ship deployments, 13
specific geographic areas were identified
Figure 19. Zones of environmental severity affecting marine
as having differing levels of environmen-
trafficability in Alaskan waters. The numbers from 1 through 13
tal severity in terms of trafficability (Fig.
represent increasing degrees of navigation difficulty in the areas
19). These zones are based on representa-
indicated (after Voelker 1990).
28
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